1. Field of the Invention
The present invention relates to an electrospray mass spectrometer and an ion source therefor.
2. Description of Related Art
An electrospray mass spectrometer using a soft ionization method has been proposed. In particular, a sample in solution is pumped from a liquid chromatograph (LC) or held in a solution reservoir. The sample is sent to a metallic capillary and drawn into it by pressure applied by an LC pump or by capillarity. A high voltage of several kilovolts is applied between the capillary and a counter electrode of the mass spectrometer to produce an electric field between them. The sample in solution in the capillary is electrostatically sprayed as charged droplets by the action of the electric field. The droplets are dried or cooled and guided into the mass spectrometer where they are analyzed.
This electrospray mass spectrometer provides a very soft ionization method in that neither application of heat nor bombardment of high-energy particles is used in ionizing sample molecules. Therefore, polar biopolymers, such as peptides, proteins, and nucleic acids, can be easily ionized as multiply charged ions almost non-destructively. Furthermore, they are multiply charged ions and so those which have molecular weights of more than 10,000 can be measured with a relatively small mass spectrometer. In this way, this instrument has excellent features.
Analytical methods of electrospray mass spectrometry include an analytical method using an ordinary ESI (electrospray ionization) ion source (for example, Japanese Patent Laid-Open No. 2002-15697) and an analytical method using a cold-spray ion source (for example, Japanese Patent Laid-Open No. 2000-285847). In the former method, charged liquid droplets are electrostatically sprayed. Solvent molecules form clusters around sample molecules in this spray of droplets. The solvent molecules are vaporized by heating. In the latter method, liquid droplets are formed by electrostatic nebulization or by nebulization without application of a voltage. The droplets are cooled to minimize removal of the solvent. Molecular ions with solvent molecules attached are produced. The solvent droplets are removed in a low-temperature desolvation chamber. These two methods have used with their respective dedicated ion sources. Therefore, measurements cannot be performed consecutively moving from the ESI mode to the cold-spray ionization mode and vice versa. Hence, two ion sources must be used. This increases the cost of the equipment. In addition, the analysis is complicated.